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1
Bring water to a boil in a 1- to 1 1/2-quart heavy saucepan, then stir in kasha. Cover and reduce heat to low, then cook until kasha is tender and water is absorbed, about 10 minutes. Transfer to a bowl and cool.
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2
Break one third of mushrooms into a food processor and pulse until finely chopped, then transfer to a bowl. Repeat with remaining 2 batches of mushrooms, transferring to bowl.
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3
Cook onion and bell pepper in butter in a 12-inch heavy skillet over medium heat, stirring occasionally, until softened, about 5 minutes. Add chopped mushrooms, garlic, 3/4 teaspoon salt, and 1/2 teaspoon pepper and cook over medium-high heat, stirring occasionally, until any liquid mushrooms give off is evaporated and mushrooms begin to brown, 8 to 10 minutes. Transfer mixture to a large bowl, then stir in kasha, parsley, soy sauce, and 1/2 cup bread crumbs until combined well. Cool 10 minutes, then stir in egg until combined well.
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4
Line a platter with foil. Spread remaining cup bread crumbs in a shallow baking dish.
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5
Form one fourth of mushroom mixture (about 3/4 cup) into a 3/4-inch-thick patty (3 1/2 inches in diameter), then dredge in bread crumbs, knocking off excess, and transfer to platter. Form and dredge 3 more patties, transferring to platter. Chill patties, loosely covered with plastic wrap, 1 hour.
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6
Heat oil in cleaned 12-inch heavy skillet over medium-high heat until it shimmers, then fry patties, turning over once, until deep golden, about 4 minutes total. Transfer patties to paper towels to drain (patties will be soft).
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7
Meanwhile, whisk together mayonnaise and chipotle sauce.
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8
Spread bread with chipotle mayonnaise and sandwich each mushroom patty between 2 slices.
